526 29 OCTOBER 2021 • VOL 374 ISSUE 6567 science.org SCIENCEPHOTO: LANCE HAYASHIDA/CALIFORNIA INSTITUTE OF TECHNOLOGYD
ive among the kelp forests of the
Southern California coast and you
may spot orange puffball sponges
(Tethya californiana)—creatures
that look like the miniature
pumpkins used for pies. No re-
searchers paid them much mind
until 2017, when William Joiner, a
neuroscientist at the University of
California (UC), San Diego, decided to look
into whether sponges take naps.
That’s not as silly a question as it seems.
Over the past few years, studies in worms,
jellyfish, and hydra have challenged the
long-standing idea that sleep is unique to
creatures with brains. Now, “The real fron-
tier is finding an animal that sleeps thatdoesn’t have neurons at all,” says David
Raizen, a neurologist at the University of
Pennsylvania (UPenn) Perelman School of
Medicine. Sponges, some of the earliest an-
imals to appear on Earth, fit that descrip-
tion. To catch one snoozing could upend
researchers’ definition of sleep and their
understanding of its purpose.
Scientists have often defined sleep as tem-
porary loss of consciousness, orchestrated
by the brain and for the brain’s benefit. That
makes studying sleep in brainless creatures
controversial. “I do not believe that many
of these organisms sleep—at least not the
way you and I do,” says John Hogenesch, agenome biologist at Cincinnati Children’s
Hospital Medical Center. Calling the rest-
ful, unresponsive state seen in jellyfish and
hydra “sleeplike” is more acceptable to him.
But others in the field are pushing for
a much more inclusive view: that sleep
evolved not with modern vertebrates as
previously assumed, but perhaps a half-
billion years ago when the first animals
appeared. “I think if it’s alive, it sleeps,”
says Paul Shaw, a neuroscientist from
Washington University in St. Louis. The
earliest life forms were unresponsive until
they evolved ways to react to their environ-
ment, he suggests, and sleep is a return to
the default state. “I think we didn’t evolve
sleep, we evolved wakefulness.”Evidence from evolutionarily ancient creatures is revealing
that sleep is not just for the brain
By Elizabeth Pennisi
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